The present invention relates to power semiconductor devices with increased turn-off current ratings, and, more particularly, to such power devices that are multicellular in construction.
Metal-Oxide-Semiconductor Turn-Off Thyristors (MOSTOTs) are power thyristors that include MOS-type turn-off structure for removing current carriers from an inner base region of the device, and, accordingly, that have gate turn off capability. High current MOSTOTs are multicellular in construction; that is, the active, or current-conducting, region of such a MOSTOT is comprised of many cells, or repeated structures. A typical MOSTOT, of one square centimeter (active device area), contains several thousand cells.
Each cell of a typical multicellular MOSTOT contains substantially identical MOS-type turn-off structure capable of turning off the same current at the same density. Conventional fabrication procedures for MOSTOTs, however, result in the peripheral cells of the active region having a higher on-state current density than interior cells, due to the effect of current spreading in a voltage-supporting layer of the device that surrounds the peripheral cells.
To achieve turn-off of the MOSTOT requires that all the cells of the active region turn off at approximately the same time. Otherwise, device current tends to flow to a cell that has not turned off, due to the lower resistance path offered by such cell. Peripheral cells are typically in this category, due to their higher on-state current density, and, consequently, conduct current at an even higher current density after device turn-off is initiated. The peripheral cells may become overloaded in that they cannot turn off peripheral cell current, and, in such a case, gate turn-off of the MOSTOT is lost. As a remedial measure, the maximum turn-off current rating of the MOSTOT could be reduced. This, however, underutilizes the MOSTOT and requires more device area for a specified turn-off current rating. It would thus be desirable to provide a MOSTOT having an increased turn-off current rating.
Other types of power semiconductor devices have reduced turn-off current ratings due to peripheral cells of their active regions having a higher on-state current density than interior cells of the active regions. Such devices include insulated-gate transistors (IGTs), gate-turn off thyristors (GTOs), and bipolar transistors. It would be desirable to increase the turn-off current ratings of these other power semiconductor devices also.